1. Field of the Invention
The present invention relates to a process for the purification of bisphenols.
2. State of the Art
The purification of bisphenols by adduct crystallization has been practiced for some time with considerable variation. U.S. Pat. No. 2,791,616 discloses a process for crystallizing a bisphenol adduct and extracting the bisphenol with added water but this requires excessive amounts of water to produce increased yield. U. S. Pat. No. 3,936,507 discloses the crystallization of dried crude bisphenol with certain solvents in certain ratios. U.S. Pat. No. 4,192,955 discloses a process for crystallizing a bisphenol from a solution in a solvent in which the bisphenol is soluble and which is miscible with water. U.S. Pat. No. 3,535,389 discloses crystallization from a two-phase mixture containing water and an organic solvent. U.S. Pat. No. 4,294,993 discloses treating bisphenol adduct with toluene or toluene and water, heating and cooling but toluene is too miscible with phenol to serve as an inert diluent coolant. Russian patent No. 798,085 discloses the use of an organic solvent added prior to condensation, such as 50% toluene or benzene and optionally residual water. U.S. Pat. No. 4,209,646 describes a process for crystallizing a hot bisphenol adduct from a solution thereof in aqueous phenol by evaporating water to cool the liquid mixture at very reduced pressure to a point corresponding to the vapor pressure of the reaction mixture but this requires impractical amounts of energy to lower the pressure to the necessary very low level. Also, the patent states that the water content is critical and should be between 2-12% weight and, preferably, 4-8% weight. U.S. Pat. No. 4,492,807 discloses a process for crystallizing a bisphenol adduct from a liquid mixture which includes water and an organic liquid which will not react with the bisphenol or phenol and is a solvent for dissolving a substantial portion of the impurities or by-products resulting from the formation of the bisphenol. The process allows crystallization at lower temperature. However, some of the solvents would likely introduce impurities and Applicants have found that the use of solvents of the patent to remove impurities did not lead to an increase in purity, size or processability of the adduct crystals.
Despite the many varied processes, including those described above, there is still a need to develop a process for the recovery of a bisphenol which provides high purity crystals of bisphenol adduct which are large and firm so that they are easier to wash and otherwise work up because they do not break in handling. It is also desirable to reduce the use of pump-circulated surface heat exchange systems and the problems associated with such surface heat exchange systems, including fouling, breaking of crystals, and induced nucleation of the crystals making them difficult to filter.